1. Field of the Invention
The invention concerns a tire provided with a capacitative sensor, a tire provided with a deformation sensor, a deformation sensor and a method for evaluating the deflection of a tire. More precisely, the invention concerns a tire provided with a capacitative sensor comprising two substantially parallel electrodes, the capacitative sensor being carried by a sidewall of the tire.
2. Related Art
“Deflection of a tire” means the amplitude of the vertical deformation of the tire under load. Measurement of the deflection allows determination of the approximate load on the tire at a given inflation pressure. The load on a tire is an important parameter, because exceeding the maximum recommended load can adversely affect the endurance of the tire. In the case of large tires, for example tires fitted on heavy vehicles or construction machinery, the maximum load for which the tire is designed is generally used to indicate loading limits.
During a rolling phase, the tire is subjected to forces in three directions:                a vertical or radial force under the effect of the load imposed by the vehicle,        a horizontal force orientated along the rolling direction, also called the ortho-radial shear force, produced by a torque applied to the tire, for example due to the acceleration of the vehicle, and        a horizontal force perpendicular to the rolling direction, also called the axial shear force, produced by making the tire drift, for example when the vehicle is rounding a bend.        
At a given point of the tire the ortho-radial direction is the direction perpendicular to the axis and to a radius passing through the point.
From the prior art, in particular from U.S. Pat. No. 6,958,615 (counterpart to WO 02/057711), which is hereby incorporated by reference, a tire is already known which is provided with a capacitative sensor comprising two substantially parallel electrodes arranged radially on a sidewall of the tire. The capacitance value of such a sensor varies as a function of the distance between the two electrodes. Consequently, by fixing such a sensor on a sidewall of the tire, the signal furnished by the sensor is a function of the deformations of the sidewalls of the tire.
The sensor of the prior art is equally sensitive to the three above forces to which the tire is subjected. In effect, the variations of the signal furnished by the sensor due to each of the three forces are of the same order of magnitude.
Thus, the signal depends on the elementary contributions of each force. Current methods then enable the circumferential extension of the tire to be determined, for which the values of the forces on the tire and of its deflection are deduced empirically, in particular by neural networks.
These methods give satisfactory results, but they are complicated to put into practice. Thus, in applications in which only a measurement of the deflection of the tire is needed, the known methods are not appropriate.
Patent application EP 1186853 relates to an arrangement that measures the sidewall deformations by means of sensors external to the tire. Two magnitudes seem accessible from such an arrangement: the deradialisation of the carcass cords that reinforce the sidewalls of a tire, and the distance between the sensors and the sidewall. No application to measurement of the deflection is mentioned in the document, and in the objective section, no precise limitation is given concerning the position of these external sensors.
The purpose of the present invention is to overcome these drawbacks by providing a tire with a capacitative sensor which enables the deflection of the tire to be measured in a simple way.